The shells of multiwall pressure vessels are made by helical coiling of a steel strip onto a central tubular member in such a way that each turn of the steel strip is wound in an opposite direction with respect to a preceding layer, and the turns of the steel strips are welded together only in a few outside layers. The strength of pressure vessels made by this method is badly affected by the force of friction of the steel strip. Hence, the layers of the steel strip should be a tight fit with one another.
To attain a tight fit of the steel strip layers with one another in the course of shell coiling is a complicated problem whose solution involves accurate folowing of the coiling pitch, snug fitting of the steel strip while coiling, ensuring against flanging outwards of the steel strip edges, which arises from the marginal bending effect when coiling the steel strip round a central tubular member.
One prior-art device for helical winding of pressure vessel shells is widely known (cf. USSR Inventor's Certificate No. 517,440, published in the Bulletin "Discoveries, inventions, industrial designs and trade marks" No. 22, 1976 (in Russian). The device comprises (a trolley traversable along a rail-truck and carrying a table and a swivel frame, which in turn mounts expansible detachable yokes with pressing rollers. The swivel frame carries also a mechanism for tensioning the steel strip being coiled. The pressing rollers appear as rolls situated on both sides of a vertical diameter of the vessel shell being coiled to press it from both sides. The roll axis makes up an angle with the generating line of the pressure vessel cylindrical shell being coiled, which angle equals the angle of coiling of the steel strip.
The mechanism for tensioning the steel strip being coiled adapted for more tight and snug laying of the steel strip layers in the course of coiling, is made as two plates between which the steel strip being coiled is drawn through.
The above-described device, however, fails to provide a snug fit of the steel strip to the central tubular member and to the below-laid steel strip layers in the course of coiling. This is due to the fact that the roll-shaped pressing roller is pressed against the generating line of the pressure vessel cylindrical shell at a single point only, as it is turned to the pressure shell generating line at an angle equal to the angle of coiling. Thus, the steel strip being coiled is pressed against the shell at a single point rather than across the entire width. As a result, the steel strip is laid on to the central tubular member or on to the below-laid layers not snugly, except for a narrow belt where the strip is pressed tightly. Apart from this, when coiling a steel strip onto the central tubular member of a pressure vessel shell, the edges of the steel strip being coiled are flanged outwards past the pressing mechanism if viewed along the direction of the steel strip run. Thus, the mechanism for tensioning the steel strip being coiled, made up of two plates fails to provide uniform tensioning of the steel strip involved.
The outward flanging of the steel strip results in that some gaps appear when coiling a next layer of the strip, whereby the tightness of fitting of the layers of the steel strip being coiled to one another is badly affected.